UNU-INWEH: Water-Health Course
Case study- CONSTRUCTED WETLANDS
SUMMARY
Wastewater treatment presents challenges in every corner of the globe as man struggles to
prevent the discharging of wastes into surface waters in order to minimize potentially damaging
environmental problems. Sustainable wastewater treatment is associated with low energy
consumption, low capital cost, and, in some situations, low mechanical technology requirements.
Natural wetland systems have often been described as the “earth’s kidneys” because they filter
pollutants from water that flows through on its way to receiving lakes, streams and oceans
(http://water.epa.gov/type/wetlands/outreach/upload/ConstructedW.pdf). These are however not
always present in locations where wastewater may be freely flowing into surface waters.
Constructed wetlands on the other hand are treatment systems designed and constructed by
engineers and scientists to provide the same outcomes as natural wetlands, and utilize natural
processes that incorporate vegetation, soils, and their associated microbial groupings to improve
water quality. They thus present the flexibility of construction where needed. Wetland treatment
systems thus provide efficient alternatives to conventional treatment systems, especially for
small communities, typically rural or suburban areas, due to low treatment and maintenance
costs (Scholz et al. 2002).
The case study presented here is of Mt. View Sanitary District (MVSD) of Martinez, California,
a community led by an independent-minded Board of Directors and a forward-thinking engineer,
who created the first wastewater wetlands on the West Coast. The project saved the rate payers
millions of dollars and established a valuable wildlife habitat in the process.
INTRODUCTION AND BACKGROUND
Wetlands, which can be natural or man-made, are recognized as providing many benefits,
including food and habitat for wildlife, water quality improvement, flood protection, shoreline
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erosion control, and opportunities for recreation and aesthetic appreciation (US EPA 1993,
http://www.epa.gov/owow/wetlands/pdf/ConstructedWetlands-Complete.pdf). They contain a
great diversity of plant and animal species and are of significant importance in locations where
they are located. According to the Ramsar Convention on Wetlands; “Wetlands are areas of
marsh, fen, peat or water, whether natural or artificial, permanent or temporary, with water that is
static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at
low tide does not exceed six metres."
(http://www.wetlands.org/Whatarewetlands/tabid/202/Default.aspx). Water filtration is one of
their greatly notable functions. Due to the presence of fauna and flora, water slows down as it
flows through, causing many of the suspended solids to become trapped and settle out. Other
pollutants are transformed to less soluble forms taken up by plants or become inactive. Wetland
plants also foster the necessary conditions for microorganisms to live there by providing the
structure needed for biofilm bacteria to process wastewater
(http://californiaagriculture.ucanr.edu/landingpage.cfm?article=ca.v065n02p73&fulltext=yes).
Through a series of complex processes, these microorganisms also transform and remove
pollutants from the water
(http://water.epa.gov/type/wetlands/outreach/upload/ConstructedW.pdf). The treatment
efficiencies of wetlands vary depending on the wetland design, type of wetland system, climate,
vegetation, and microbial communities (Vacca et al. 2005).
CONSTRUCTED WETLANDS
Constructed wetlands treatment systems are secondary treatment facilities, which generally fall
into one of two categories of either Subsurface Flow Systems or Free Water Surface Systems.
Both types of wetlands treatment systems typically are constructed in basins or channels with a
natural or constructed subsurface barrier to limit seepage (USEPA)
Subsurface Flow Systems (SF) (Figs 1a and b).
http://ucce.ucdavis.edu/files/repository/calag/fig6502p75thumb.jpg)
(http://californiaagriculture.ucanr.edu/landingpage.cfm?article=ca.v065n02p73&fulltext=yes).
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Subsurface Flow Systems are designed to create subsurface flow through a permeable medium,
keeping the water being treated below the surface, thereby helping to avoid the development of
odors and other nuisance problems. Such systems have also been referred to as "root-zone
systems," "rock-reed-filters," and "vegetated submerged bed systems." The media used (typically
soil, sand, gravel or crushed rock) greatly affect the hydraulics of the system. (US EPA). SF
wetland systems are best suited for small to moderate sized applications and at larger systems
where the risk of public contact, mosquitoes, or potential odors is a major concern. SF wetlands
remove BOD (biochemical oxygen demand), COD (chemical oxygen demand), and TSS (total
suspended solids), and with sufficiently long retention times can also produce low levels of
nitrogen and phosphorus (When too much nitrogen and phosphorus enter the environment usually from a wide range of human activities - the air and water can become polluted).
Free Water Surface Systems (FWS). (Fig 2).
http://www.sswm.info/category/implementation-tools/wastewater-treatment/hardware/semicentralised-wastewater-treatments/f
Free Water Surface Systems are designed so that the water surface is exposed to the atmosphere.
In this system, vegetation is planted in base soils below water as deep as 4 feet (Fig. 2,
http://californiaagriculture.ucanr.edu/landingpage.cfm?article=ca.v065n02p73&fulltext=yes).
They are designed to simulate natural wetlands, with the water flowing over the soil surface at
shallow depths. Most natural wetlands are FWS systems, including bogs (primary vegetation
mosses), swamps (primary vegetation trees), and marshes (primary vegetation grasses and
emergent macrophytes.).
The influent (Influent streams do not usually get deeper and wider downstream) to these
wetlands spreads over a large area of shallow water and emergent vegetation. The subsequent
low velocity and essentially laminar flow provides for very effective particulate removal in the
front part of the system. This particulate material, characterized as total suspended solids (TSS),
contains Biochemical Oxygen Demand (BOD) components, fixed forms of total nitrogen (TN)
and total phosphorus (TP), and trace levels of metals and more complex organics. The oxidation
or reduction of these particulates releases soluble forms of BOD, TN, and TP to the wetland
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environment, which are available for adsorption by the soils and removal by the active microbial
and plant populations throughout the wetland (USEPA 2000).
WASTEWATER WETLANDS, MT. VIEW SANITARY DISTRICT, MARTINEZ
CALIFORNIA (FWS)
Mt. View Sanitary District (MVSD) provides wastewater treatment for approximately 16,000
people living in and around Martinez, California. This community, led by an independentminded Board of Directors and a forward-thinking engineer, created the first wastewater
wetlands on the West Coast. The Mt. View Sanitary District marshes are located in an urban
environment and the marsh is bisected by an interstate highway. The project saved the rate
payers millions of dollars and established a valuable wildlife habitat in the process.
In 1974 the District began with a simple 10-acre wetland divided into two sections. The area that
was created by scraping away the topsoil became a shallow, open-water pond. The other area,
whose topsoil was not disturbed, was quickly colonized by emergent vegetation, such as cattails.
In 1977 the marsh was expanded to include 10 more acres of land divided into three marsh areas.
One was constructed as an open-water pond with islands to provide protected nesting habitat for
waterfowl. A second marsh was seeded with plants to provide food for waterfowl, such as water
grass and alkali bulrush (Echinochloa crusgalli and Scirpus robustus). The third area was
designed in a serpentine fashion to provide maximum water/plant contact to enhance treatment
effectiveness.
The next 22 acres, added to the marsh system in 1984, were located across the interstate to the
north. This area had been seasonally flooded and the District merely had to make minor changes
to water control structures to allow the marsh's inclusion in the system. The most recent addition
to the wastewater wetland complex is a 43-acre section that also is located to the north of the
interstate and adjacent to the previous 22 acres. The wetlands area totals 85 acres. This bountiful
wildlife habitat includes plants, animals, fish and invertebrates. Some of the animals are
permanent residents of the marshes, while others are temporary visitors that stop along their
migratory journey. Plants grow in the marshes as well as on the levees surrounding the marshes
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and a riparian corridor (a unique plant community consisting of the vegetation growing near a
river, stream, lake, lagoon or other natural body of water) is beginning along Peyton Slough.
There are emergent plants rooted in the bottom muds as well as submerged plants.
Mt. View Sanitary District provides secondary treatment to approximately 1.3 million gallons
per day of wastewater from approximately 16,000 residents in the Martinez, Calif., area.
Although there is some light industry and commercial development within the District's service
area, the primary source of the wastewater is residential. The District maintains strict
pretreatment standards and prohibits the discharge of heavy industrial waste into its sewerage
system. (USEPA 1993)
FINDINGS
According to USEPA 1993:
Wetland plants provide food and shelter for marsh biota and improve water quality. Birds,
mammals, reptiles and amphibians eat plant leaves, seeds and roots of the more than 70 species
of marsh and riparian vegetation. Dense growths of marsh bulrushes provide nesting sites for
songbirds as well as ducks. The most visible animals at the marshes are the more than 123
species of birds and more than 15 species of birds that nest in the wetland.
Although the primary purpose for constructing the wetland is to create wildlife habitat, it also
improves water quality for some parameters. There are numerous processes by which plants
contribute to water quality improvements, including direct uptake of nutrients by algae and some
rooted vegetation. The plants foster settling of particulate matter by slowing water movement
and greatly increase the contact with microorganisms that live on the surfaces of emergent
plants. These microorganisms metabolize pollutants, decreasing their dissolved concentrations in
the water. Monitoring shows that wetland nutrient concentrations follow a stable seasonal cycle
that varies little from month to month, but clearly shows a difference between the cold, wet
season (November through April) and the warm, dry season (May through October).
The concentration of nitrates decreases in the wetland during the summer months. There is
limited evidence to suggest that the wetland is removing cadmium, copper, silver and zinc. In
addition, periodic special monitoring studies are undertaken to answer specific questions
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concerning the processes or biota within the wetlands. Studies at the marsh have included an
ammonia study and a fisheries and benthic invertebrate study. Doubtless the largest special
study, however, occurred after the 1988 spill of 440,000 gallons of crude oil into the marsh from
an adjacent refinery. The cleanup efforts included picking up oily water by vacuum trucks,
rototilling of contaminated soils and hand-cutting vegetation in less inundated areas of the marsh.
The recovery of the marsh's vegetation and soils was monitored closely and eight months later
this section of the wetland resumed operation.
CONCLUSION
It is clear that constructed wetlands water treatment projects are a positive endeavor, that both
improve the quality of wastewater flowing through them and provide habitat/feed wildlife that
come to the wetland. They are ecosystems that have evolved to separate the toxins in their water
supply and use the elements with nutritional value to grow vegetation and keep the environment
healthy. Constructed wetlands are good for the community and environment, provided they are
properly constructed and guidelines followed. There are some down sides to the constructed
wetlands, i.e. their performances vary depending on the season (better performance in the
summer and less in the winter) and they also need several years (i.e. 5 years) of operation to
obtain maximum efficiency.
REFERENCES CITED
1. http://water.epa.gov/type/wetlands/outreach/upload/ConstructedW.pdf .
2. Scholz M, Höhn P, Minall R (2002) Mature experimental constructed wetlands treating
urban water receiving high metal loads. Biotechnol Progr 18:1257–1264
3. US EPA, 1993. http://www.epa.gov/owow/wetlands/pdf/ConstructedWetlandsComplete.pdf
4. (http://www.wetlands.org/Whatarewetlands/tabid/202/Default.aspx).
5. Gabriela Vacca, Helmut Wand, Marcell Nikolausz, Peter Kuschk, Matthias Kästne .
2005. Effect of plants and filter materials on bacteria removal in pilot-scale
constructed wetlands. Water Research. Volume 39, Issue 7, Pages 1361-1373
6. Tilley, E.; Luethi, C.; Morel, A.; Zurbruegg, C.; Schertenleib, R.
(2008): Compendium of Sanitation Systems and Technologies. Duebendorf,
Switzerland: Swiss Federal Institute of Aquatic Science and Technology.
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(Accessed. http://www.sswm.info/category/implementation-tools/wastewatertreatment/hardware/semi-centralised-wastewater-treatments/f)
IMAGES
Fig. 1a. Subsurface Flow System
(http://ucce.ucdavis.edu/files/repository/calag/img6502p73athumb.jpg)
Fig. 1b. Subsurface Flow System
(http://ucce.ucdavis.edu/files/repository/calag/img6502p73athumb.jpg)
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Fig 2. Schematic diagram of a free-water surface constructed wetland. Source: TILLEY et al.
(2008)( http://www.sswm.info/category/implementation-tools/wastewatertreatment/hardware/semi-centralised-wastewater-treatments/f)
Fig. 3. Martinez, San Francisco Bay Area, California. (http://en.esstatic.us/upl/2012/10/San_Francisco_Martinez.jpg)
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